A light emitting diode (LED) is a semiconductor that gives off light when current is passed through it. The light emitted by these elements resembles most incandescent or compact fluorescent (CFL) light bulbs but may be preferred because they consume less electricity and last longer. In addition, compared to incandescent or CFL bulbs, LED bulbs are generally smaller in size, are more durable and reliable, and can better withstand extreme temperatures.
LEDs were first designed to be used in circuits that run on low-powered DC voltage. They were often used in low intensity lighting applications, such as in an exit sign, a power button on a computer, or a light on a video camera. Subsequently, high-powered LEDs have also been developed to illuminate a larger area, such as in flashlights, light bulbs, or integrated light fixtures. However, the heat generated by these high-powered LED devices is much greater than in the low intensity lighting applications, as further explained below.
LEDs are still typically powered by a DC power source, such as a battery, where current runs in one direction only. Some lighting devices have utilized drivers or transformers between the building wiring or wall socket and the fixture to provide the required DC voltage to the lighting device. Other lighting devices, such as LED light bulbs, include built-in drivers or transformers for performing such a function. The need for a driver or transformer can be very space consuming, especially if required to fit within a standard light bulb encasement. Additionally, the additional component of a driver or converter renders the electrical components less stable and more prone to failure. The heat derived from the use of a driver or converter often puts the components at greater risk for electrical failure.
However, a new approach for lighting has been the development of AC-LEDs, which can operate directly from an AC power supply. These AC-LEDs can handle a higher voltage and can accept an AC voltage directly, without AC to DC conversion. Several methods of creating the AC-LEDs have been employed. For example, a method developed by Seoul Semiconductor takes two strings of series-connected die, connected in different directions whereby one string is illuminated during the positive half of the AC cycle, the other during the negative half. The strings are alternately energized and de-energized at the 50/60 Hz frequency of the AC main power source, and thus the LEDs always appears to be energized. It is recognized that other methods for making an AC-LED have been developed. Through these methods, the AC-LEDs can transmit power more efficiently and effectively, without the need for intervening electronics.
In certain lighting applications, these AC-LEDs have been employed as a primary light source when utility power is available. A lighting device employing these AC-LEDs in combination with “standard” DC-powered LEDs in the same lighting device may provide an additional benefit during a power failure. Namely, the DC-LEDs, or a portion thereof, may have power supplied from a battery to provide emergency lighting during a power outage.
It is desirable to maintain a full charge in the battery to ensure maximum duration of emergency lighting. A battery charging circuit may be provided to charge a battery from the AC power used to power the AC-LEDs. However, the addition of a battery charging circuit has its drawbacks. The battery charging circuit requires additional electronic components that take up additional space, and a portion of the electronic components includes power conversion devices that generate additional heat. In addition, operation of the AC-LEDs also generates heat. If the temperature rise is too great, the battery begins to charge less efficiently and eventually can no longer be charged. Further, attempting to charge the battery at too high of a temperature can result in permanent damage to the battery. Thus, it would be desirable to provide an improved battery charging system that can maintain charge in the battery while the AC-LEDs are operational without overheating.